Seafastening a Grillage - Beam Analysis Model

[PieRRound]

Howdy friends.

I have designed many a grillage in my day, as well as the seafastening for said grillage.

Typically I use RISA 3D to determine the reactions at the locations of my seafastening cleats; I design my cleats and welds accordingly.

I recently got into a bit of a debate (as is common among engineers) regarding the proper boundary conditions for a seafastened grillage on the back deck of a pipelay vessel.

In the midst of all of these great minds (present company excluded), we actually reached a bit of a stalemate.

There seemed to be valid arguments from both camps...

So without further adieu - I offer this topic of discussion to THE FORUM.

If you were to model a grillage in RISA 3D or STAAD, how would you model the under-deck members of the vessel underneath the beams of the grillage - keeping in mind that there will be seafastening cleats welded to the grillage beams and to the deck -- does this constitute a FIXED boundary condition at the location of the seafastening cleats?

Needless to say, the boundary conditions specified in the model will have an influence on whether or not the grillage beams pass for bending and shear checks (based on the operational loads of the equipment that is on top of the grillage.

Yes, statically indeterminate... It's a valid point.

Thoughts? Fixed boundary conditions OR simply supported?

 

[dhengr]

PieRRound:
What a description of your problem, if you’re you and sitting right there and can see all the details and conditions. But, we can’t see them from here, so the description is seriously lacking in detail. Are all grillages and sea fastening cleats identical, no variations in any way? I use my grill on the back of my vessel, and things turn out pretty good too. I’ve laid some things on my boat too, but I have never needed RISA. We must be talking about two different things. You are probably talking to a fairly small group unless you furnish a few sketches, details, photos, with loads, dimensions, material sizes, proper proportions in the sketches, etc. Then, you tell us why you think the sea fastening cleats are or aren’t fixed, and we’ll comment.

 

[PieRRound]

Noted. A sketch would definitely help.

In the attached sketch I have drawn a basic grillage... let's assume it is made of W sections. The main reason for the grillage is to ensure that the steel deck of the vessel is not over-stressed - we're spreading loads out to the main frames of the vessel.

Underneath the grillage are the underdeck frames of the vessel. These frames are usually evenly spaced, and in most cases they run transversely and longitudinally.

There will be a piece of equipment bolted to the grillage.

Worst case vertical reaction forces (dynamic loads) where the equipment is bolted to the grillage are calculated based on the equipment's operational loads combined with vessel motions (heave, pitch, roll, etc).

The grillage beams are seafastened to the deck with steel plate cleats welded to the deck and to the beam.

In order to make sure the grillage is fit for purpose, we use hand calcs and RISA.

The question is whether or not the boundary conditions in RISA should be defined as FIXED at the locations of the seafastening cleats - OR - simply supported at the locations where the grillage beams sit on the vessel frames.

Hope that helps explain things a bit better.

 

[MikeHalloran]

There are instructions around here for attaching files; you need to find and read up on that.

My $.02:

No part of a boat is fixed, in any sense of the word.




Mike Halloran
Pembroke Pines, FL, USA

 

[PieRRound]

I thought I had attached the sketches properly, but I guess not. I'll dig around to figure out what went wrong.

And yes, no part of a boat is fixed. I think my sketches will help to make things a bit more clear.

 

[dhengr]

PieRRound:
It must a real bitch to fabricate off of your Etch A Sketch drawings. If you don’t know what an engineering sketch or drawing really is, you probably shouldn’t be pretending to be doing engineering, even if you do have RISA. You’re talking to real engineers and tech people here, so put the crayons away. If you don’t understand how important proper proportions are in you sketches, how dimensions, loads, member sizes, details, details, etc. etc. influence how people look at your sketch, and the initial assumptions they make, based on what you’ve shown them, you won’t get answers any better than your sketches. What I see in the second sketch is a couple .25" thk. plates chicken sh.. welded to a W 14x665, and that’s not fixed. The first sketch shows a grillage, maybe, but we have no idea what level the various beams are at, how they are attached, their relative sizes, why you selected those hold-down locations on your grillage/skid. All of these things influence each other, that’s part of what design is all about. You really should take this problem to your boss, so he knows what you know and don’t know, and can keep you and the company out of trouble. Having a local mentor who can help you and guide you, and be looking at the same equipment and drawings, etc. is really important in your development. Someone once said... “I personally think it would be a piece of cake for any savvy engineer.” This problem is not a piece of cake and you’re not showing much savvy at the moment.

 

[PieRRound]

I was unaware that this forum required formal engineering sketches... what you got was quickly thrown together in PAINT while sitting at my breakfast table. To be honest, I would have LOVED an Etch A Sketch.

A conversation around a white board with dry erase markers would have yielded some similarly "proportionate" sketches.

I appreciate the constructive criticism.

 

[rb1957]

it doesn't "require" anything, but the better the sketches (and sorry but yours are pretty bad at relaying info) then the better we understand the problem.

here's what i got from your sketches (and i hope i didn't get a virus for the "funky" site you posted on (there's an easy way to link a pic on your computer directly to your post, at the bottom of the screen) ...

i get the grillage is distributing load to the vessel.

it looks like at two points you have a clamp type fttg, and at three point (the green ones) you're applying vertical load ... that's my guess 'cause i'd expect a much more distributed set of vertical reactions into the vessel frames.

if this is your setup, then you seem to be deliberately avoiding taken one of the moments through the clamp fttgs, the moment along their axis. if you've got multiple vertical reactions then you've plenty of load paths for taking moment out of the grillage, and don't need to rely on fixed supports. fixing the supports would be conservative on the supports and their local structure, pinning the clamp fttgs would relieve a load that may well develop (and cause local plasticity before the grillage breaks ... what i mean is that in the ultimate load situation these local fixities don't absorb much of the load and will relieve/limit themselves by plasticity). there's going to be some clearance in the clamps, yes?, so initially pinned, then fixed, then plastic hinge.

Quando Omni Flunkus Moritati

 

[dhengr]

PieRRound:
The point isn’t that you need to show perfectly scaled engineering/fabricating drawings and details, but you should try to convey some real engineering information, or you are wasting your and our time. Show some of the detailed info. that goes into the design, if you want an intelligent discussion about the design. A W18 beam will look different than a W8 beam and that’s important as to who’s (which beam) pulling who around, and which frames into which. Top of beam elevations are important as to how they frame together. A W18 beam which cantilevers 2' will look and act quite differently than one that canti’s. 8', and that length and depth difference are important in the way we look at that beam. You would be surprised at the fairly quick initial judgements/impressions experienced engineers, who aren’t solely dependant on RISA for their judgement, will make about your problem, based on your drawing proportions and their experience. And, you’ll leave them guessing or have them going 180̊ in the wrong direction if you don’t convey meaningful info. There are some very smart and experienced engineers here on E-Tips, and they are more than welling to spend their time to help you, but don’t waste their time with those kinds of sketches. We can guess that much, but we don’t like the 20 questions game either. Scans of real, partially complete, drawings and details are much better than what you showed. Sufficient dimensions, loads, details, cross-sections, etc. are important so we have a good feel for the scope and order of magnitude. When you show a beam section as you did, you must be talking about a very heavy W14, because they are the only sections with those proportions, whether you know it or not. Good sketching is the way experienced engineers communicate, and you better do better than above even on your white board in the office, if your intention is to convey meaningful info.